Reddit reviews Modern Mar:radio:out Of Thin

It will not pop as well as if you had it the other side down. There is a special pad that absorbs the microwaves, heats up, and evenly distributes the heat to the oil, butter, and popcorn mixture. It makes the process more efficient.
Found a bit in Wikipedia:
>A susceptor, usually a metalized film laminated onto the paper of the bag, absorbs microwaves and concentrates heat at the film interface, thus ensuring a heat distribution focused on the hard-to-heat flavor coating so that the unpopped kernels are evenly coated prior to popping, thereby ensuring even flavor throughout the product. Additionally, some popcorn is flawed and will not pop because of possible damage to the shell, which allows the steam to escape.

I also remember watching a Modern Marvels episode that contained a segment about this.
here is one episode:
probaly this one
Or this one
dont think its this one

The microwaveable popcorn was invented by General Mills.
An early susceptor popcorn bag design was patented by the American company General Mills in 1981, US Patent #4,267,420